Electric valve converting system



June 24, 1941. w HULL 2,247,057

ELECTRIC VALVE CONVERTING SYSTEM Filed May 20, 1939 Fig. I.

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Patented June 24, 1941 ELECTRIC VALVE CONVERTING SYSTEM Albert W. Hull, Schenectady, General Electric Company,

New York N. Y., assignor to a corporation of Application May 20, 1939, Serial No. 274,775

6 Claims.

My invention relates to electric valve converting systems and particularly to such systems wherein a plurality of series connected discharge devices are utilized, and has for its principal object to provide simplified and improved means for controlling the potential distribution among the series of discharge devices.

For very high voltage installations where electric Valve converting systems are used it has sometimes been found desirable to operate a plurality of electric valves in series whereby at least two important advantages are obtained. In the first place each individual discharge valve need not be designed to withstand the full voltage of the system, and in the second place much greater reliability of operation is obtained, for in the present state of development of commercial electric discharge valves one cannot be sure that the individual valves will never fail. However, by using a number of electric discharge valves in series the probability of failure of the system can be greatly reduced. For, even though any one electric discharge valve should fail, the strain on the remaining serially connected valves would be increased only by a negligible amount and the probability of failure of the remaining valves would be very remote. When such valves are connected in series and employed in high voltage installations considerable difiiculty has been encountered owing to the fact that because of differences in the valves themselves and also diiferences in their positions in the circuit they do not assume equal proportions of the voltage impressed upon them and a large proportion of the inverse voltage may appear across one of the valves causing it to flash over. If, however, some means were provided to cause the series connected valves to assume equal proportions of the voltage impressed upon them this difiiculty would be eliminated. It has been suggested that each of the serially connected valves be paralleled with a variable capacitor which is generally relatively large in comparison to the internal capacity of the discharge valves themselves, so that the voltage assumed by each of the valves may be balanced or the voltage appearing across any one valve may be made to assume any value desired. This arrangement is described and claimed in U. S. Letters Patent No. 1,867,419, granted July 12, 1932, upon an application of F. W. Peek, Jr.

It has been found that capacitors connected in parallel with each of the serially connected discharge valves may very satisfactorily cause the rapid voltage changes or transient voltages impressed on the system to be divided equally among the difierent electric valves, but such an arrangement will not cause the slow changes of voltage or steady state condition voltages to be divided equally among the several valves, since during this latter condition the capacitances act as infinite impedances and lose their voltage balancing characteristics. In accordance with my invention I have provided an arrangemet whereby the voltage may be divided equally among the serially connected discharge valves for both steady state and transient conditions.

It is an object of my invention to overcome the disadvantages above noted in connection with serially connected discharge valves for use in high voltage electric valve converting systems.

It is another object of my invention to provide a new and improved electric valve converting system.

It is a further object of my invention to provide a new and improved electric valve converting system of the type wherein a plurality of serially connected electric discharge valves are used,

In accordance with the illustrated embodiment of my invention I provide an arrangement for balancing the voltages between serially connected electric discharge valves for both steady state and transient conditions comprising a capacitance connected in parallel with each of the valves for balancing transient voltages among the various valves and a resistor constructed of resistance material having an inverse voltage-resistance characteristic connected in parallel with each of the discharge valves for dividing the steady state voltages equally among the various discharge valves.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a diagrammatic representation of an electric valve converting apparatus in which my invention has been embodied, Fig. 2 illustrates another modification embodying my invention, and Fig. 3 is a diagrammatic representation of a portion of Fig. 2 with certain modifications.

Referring now to Fig. 1 of the drawing, I have illustrated therein an electric valve converting apparatus for transmitting energy between an alternating current circuit l9 and a direct current circuit ll. Since my invention is applicable to any electric valve converting system well known in the art in which a plurality of serially connected electric discharge valves are utilized,

the arrangement shown in the drawing merely shows a half wave rectifying circuit for the purpose of simplicity of disclosure and explanation.

The primary winding of a transformer I2 is energized from the alternating current source it and the secondary winding thereof is connected at one terminal to one side of the direct current circuit and at the other terminal thereof to one of the electrodes of one of the serially connected electric discharge valves it, Hi, i5, IE, l1, l3. Each of the electric discharge valves it to E8 is illustrated as of the hot cathode type and comprises an anode l9 and a cathode 28 enclosed within an envelope. My invention is applicable for use with any of the well known electric discharge valves but I have found it to be particularly useful in connection with electric valves of the type wherein an anode and a cathode are mounted within an envelope containing an ionizable medium. The cathode 20 of electric discharge valve i8 is connected to the other side of direct current circuit 1 i. a

The heating current supply means for the cathodes 29 of electric discharge valves it to 18 comprises a plurality of transformers 2| to 26 connected in cascade arrangement, that is, a single source of electric power 2'! energizes the primary winding of transformer 2i while the primary windings of the remaining transformers 22 to 26 are each energized from the secondary winding of their adjacent transformer. By this arrangement the relative capacities .of the. transformers may be utilized to tend to equalize the capacitances of the series of electric discharge valves. A plurality of condensers 28 to 33 are connected in parallel with the valves 13 to 58 respectively. These condensers serve to balance the capacities between the various valves and hence cause a balancing of the transient voltages assumed by each of the valves. With the cathode heating transformers 2! to 2i: arranged as shown in Fig. 1 the capacitances of condensers 28 to 33, need be relatively small to balance the capacities between the various valves. different values of capacitances may be required due to the differences in internal capacities of the valves and associated apparatus, and also due to the different positions of the different valves with respect to each other and also with respect to the remainder of the apparatus. That is, each of the external capacitances has a magnitude dependentupon the'capacity of the particular valve with which it is associated as contrasted with the use of external capacitors which are so large that the internal capacity of the associated valve is negligible. In other words the external capacitance is of the same order of magnitude as the internal capacitance of the respective electric valves. It will also be understood that the windings of the cathode-heating transformers may be so proportioned as to give the required distribution of capacitance without the use of capacitors 23 to 33. In order to balance the voltages assumed by the different valves for steady state conditions, I provide a plurality of resistors 3% to 39 connected in parallel with each of the valves it to i8. Resistors 36 to 33 should preferably be constructed of a resistance material having the property of decreasing its resistance with an increase of applied voltage and such material will be referred to as having an inverse voltage-resistance characteristic. Some of the resistance material of this type has the advantage of being very compact and since It will be understood, of course, that the greater the potential difference across the resistance material the less the resistance, such material will operate very advantageously to balance not only the steady state voltages but in some cases might even be used for balancing both the transient and steady state voltages. One such resistance material of this type is commercially available under the trade name Thyrite and is described and claimed in U. S. Letters Patent No. 1,822,742, granted September-8, 1931, upon an application of Karl B. McEachron.

While in Fig. 1 I have illustrated my invention as applied to an electric valve converting system where an alternating current is converted into a direct current, it will be understood by those skilled in the art that the apparatus is equally suitable for converting direct current to alternating current. In this latter arrangement, however, .it is usually considered desirable to provide a control electrode or grid for each of the electric discharge devices. Accordingly, in Fig. 2 I have shown a plurality of three-element discharge valves Mi, ii anddZ connected in series relation. Each of 'the electric discharge valves is provided with an anode d3, a'hot cathode as, and a control electrode or grid 85. .The heating current supply means for cathodes M includes transformers 46, ll and d8 respectively energized from any suitable source of electrical energy. A plurality of grid transformers 49, 5B and iii energized from any suitable source pro-t vide the excitation potential necessary to energize control electrodes 55 which are connected in series with the secondary windings of transe formers :39 to 5! through suitable current limit-: ing resistances 52, 53, 5:23. In order to balance the transient voltages among the serially con-, nected valves 4d to 32 I provide capacitors, 55, 56, 51 connected in parallel with the respective valves. In order to balance'the steady state voltages among the several valves I provide resistors 58, 52, 6E] constructed of a resistance ma-- terial having an inverse voltage resistance characteristic connected in parallel with'discharg valves lll to Q3, respectively.

The cascade transformer arrangement shown' in Fig. 1 is also suitable for use with serially connected grid controlled electric discharge Valves as shown in Fig. 2. Accordingly, in Fig. 3.1 have shown a portion of Fig. 2 to which I have applied the cascade transformer arrangement illustrated in' Fig. l. The same reference numerals are retained for correspondingparts. In this case the grid transformers 5i! and 5! are fed from a series of cascade transformers 2i, 22', 23, similar to the transformers 2i to 26 which energize the cathode heating elements of the electric discharge'valves of Fig. 1. In Fig. 3 I have illustrated the cascade arrangement oftransformers as supplying power both to the grid transformers 5d and hi and also for heating the cathodes ie of electric discharge valves 4i and Q2. The primary winding 21 should in this case be connected to. suitable phase shifting means (notshown) for varying the phase of the alternating current, as is well known by those skilled in the art. It will be understood, of course,

that the grid transformers 59, El 'and grid transformers id to El of Fig. 2 may be arranged to produce voltages of peaked wave form and suitable biasing means may be provided in the grid to cathode circuits of both Figs.2 and 3 if desired, without departing from my invention in its broader aspectsl a A WhileI have shown and described particular embodiments of my invention as applied to an electric valve converting apparatus, it is to be understood that this is merely illustrative, as one of a number of electric valve converting or trans lating apparatus to which my invention may be applied. It will, of course. be obvious to those skilled in the art that changes and modifications may be made Without departing from my invention and I therefore aim in the appended claims to cover all such changes or modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a circuit including a plurality of electric discharge valves having the anode-cathode circuits thereof connected in series, means for impressing a voltage on said circuit, and means including a resistance niaterial having a continuous inverse voltage-resistance characteristic for distributing said voltage among said valves.

2. In combination, a circuit including a plurality of electric discharge valves connected in series, means for impressing a voltage on said circuit, means including a condenser and a resistor of resistance material having an inverse voltage-resistance characteristic connected in parallel with each of said valves for equalizing the voltages assumed by said valves.

3. In combination, a circuit including a plurality of electric discharge valves of the hot cathode type connected in series, means for impressing a voltage on said circuit, means for heating the cathodes of each of said Valves including a separate transformer for each valve, means comprising said transformers to provide a capacitanc in parallel with each of said valves for equalizing the transient voltages assumed by said valves and means including a resistor having an inverse voltage-resistance characteristic connected in parallel with each of said valves for equalizing the steady state voltages assumed by each of said valves.

4. In combination, a circuit including a plurality of electric discharge valves connected in series, said discharge valves including a control Iii electrode and a heated cathode, transformer means for energizing both said control electrode and the cathode heating means, means for impressing a voltage on said circuit, means including said transformer means arranged in cascade relation for equalizing the transient voltages assumed by said valves, and means including a resistor having an inverse voltage-resistance characteristic connected in parallel with each of said valves for equalizing the steady state voltages assumed by each of said valves.

5. In combination, a supply circuit, a load circuit, one of said circuits being an alternating current circuit, a plurality of series connected unilaterally conducting electric valves connected in series with said load circuit and across said supply circuit and each comprising an anode and a cathode mounted Within an envelope comprising an ionizable medium, said valves being alternately conductive and nonconductive at the frequency of said alternating current circuit, a condenser connected in parallel with each of said valves and each having a capacity of the same order of magnitude as the capacity of the Valve with which it is associated, the combined capacities of each of said valves and its associated condenser being substantially equal so that transient voltages are evenly distributed among said valves, and a resistor connected in parallel with each or said valves and its associated condenser for distributing voltage changes which are slow relative to the voltage changes distributed by said condensers, the magnitude 01' said resistors being large compared to the impedance of said valves to current flow in the normal direction so that substantially no current flows through said resistors during the periods that said valves conduct current in the normal direction.

6. In combination, a circuit including a plurality of electric discharge paths connected in series, means for impressing a voltage on said circuit and a resistance having a continuous inverse voltage-resistance characteristic connected in parallel with each of said valves for equalizing the voltages assumed by each of said valves.

ALBERT W. HULL. 

